Recirculation of exhaust gases has been employed to inhibit the formation and emission of oxides of nitrogen from internal combustion engines. For diesel engines, it has been proposed to vary recirculation of exhaust gases in accordance with throttle position and engine speed--increasing recirculation as the throttle reduces fuel flow, and increasing recirculation with engine speed. A vacuum regulator valve has been employed to create a vacuum signal which varies with throttle position, and a diaphragm operated exhaust gas recirculation valve has controlled recirculation in accordance with such a throttle position related vacuum signal. However, systems responsive to both throttle position and engine speed have heretofore required electronic control circuitry.